This invention generally relates to a gas evolving electrolysis cell and in particular to the arrangement of electrodes in an alkaline electrolysis cell for generation of hydrogen.
Many important industrial processes involve electrolysis operation with generation of gases from an electrolyte solution. For example, electrolysis of potassium hydroxide solution for production of hydrogen and oxygen gases is an electrolytic process involving generation of gases at electrodes. Most industrial electrolyzers have a bipolar configuration, with bipolar plates stacked in electrical series. Voltage is applied at the end plates and each intermediate plate acts as a positive electrode (anode), on one side, and a negative electrode (cathode), on another. A bipolar electrolyzer typically comprises multiple cells with parallel paths for electrolyte solution and product gases. These cells are typically fed with the electrolyte solution at the bottom of the cell. The electrolyte solution and the product gases from the electrolysis process typically exit at the top of the cell.
During electrolysis, ions are transported between the electrodes. In water electrolysis, hydrogen ions at the cathode form hydrogen bubbles and oxygen ions at the anode form oxygen bubbles. Typically a membrane is provided between the electrodes to avoid mixing of the product gases. The gas bubbles move with the electrolyte upwards and are removed as a two-phase mixture of the product gas and the electrolyte. Typically the volume fraction of bubbles in the electrolyte solution increases as the bubbles go up within the electrolyte solution. This results in a blanketing of the electrodes by the gas bubbles. This in turn increases the ohmic resistance of the electrolyte solution and the current density decreases along the electrode surface.
In general, the rate of degradation of an electrode is a function of local current density value. A non-uniform distribution of current density on the electrode surface results in high local current density values at some points on the electrode surface, where a premature degradation of electrode surface occurs. This necessitates a replacement of the electrode even when all parts of the electrode are not degraded. If the degradation occurs more frequently, the system needs to be shut down more often for replacement of the electrodes in turn increasing the operating costs.
Therefore there is a need to design electrolyzers with less degradation of electrodes and lower operating costs.